In hitherto known telecommunication networks, a subscriber terminal as a first type radio transceiver device (hereinafter: mobile station MS), in order to be operated within a network, needs to be registered to the network NW, i.e. to a network controlling device like for example a mobile services switching center MSC (or an SGSN), which controls so called base station controllers BSC, which in turn control base stations BS as second type radio transceiver devices.
To this end, each subscriber has a subscriber identity module SIM to be inserted into the used mobile station MS as a respective terminal equipment. The SIM contains a pre-stored international mobile subscriber identity number IMSI, by which a user can be identified. However, in order to protect the user against being identified by an intruder in the network, each user is assigned a temporary mobile subscriber identity TMSI. This identification which changes either from time to time or from area to area (when combined with a location area identifier LAI) allows an “anonymous” identification of the user when using his terminal.
For details of the roughly described registration procedure including ciphering of transmitted data for authentication at registration, which details are considered to be not necessarily to be described here, the reader is referred to the plurality of respective publicly available GSM specifications.
Likewise, an attached or registered subscriber or mobile station, respectively, will have to perform a detach from the network under specific conditions. For example, the mobile station will be detached from the network and its registration will be abandoned, in case the SIM module is detached from the terminal equipment or the like.
In such cases, the mobile station MS sends a detach message to the network NW, the so-called IMSI DETACH INDICATION message. Upon receipt of the IMSI DETACH INDICATION the network controlling device (MSC) sets an inactive indication for the mobile station MS, while no response is returned to the mobile station itself. (For details, also in this context it is referred to the respective GSM specifications). Namely, no authentication is conducted at detach, when the mobile station initiating the detach procedure leaves the network.
Thus, there exists a possibility that a malicious user may obstruct or even terminate a third party's call by sending detach messages with random identities of mobile stations (i.e. random numbers of TMSI identifiers). Stated in other words, although it is not possible to interrupt the connection to a specific mobile station MS of a certain specified user by sending such a detach message, a lot of damage and irritation can be caused to a great number of users as well as to the operator of the network NW, when arbitrary calls and/or radio connections are blocked and/or terminated by the intention of a malicious third party.
A previously proposed approach to prevent this resides in performing an authentication procedure when a mobile station MS is to be detached from the network NW, i.e. upon receipt of a detach message at the network from the mobile station.
However, such a proposed authentication at detach is rather time consuming in many situations and has therefore only a limited applicability.
Moreover, performing an authentication procedure may not be feasible if the mobile station is performing power off, i.e. is switched off, or the available battery power is too low so that normal operation of the mobile station can not be assured any longer.